The effect of non-uniform He plasma on the properties of thin oxide (11.5-13 nm) metal-organic-semiconductor (MOS) structures has been investigated by means of C-V and breakdown-voltage histogram techniques. The plasma reactor system used is like a magnetically enhanced reactive ion etcher. The damage creation for MOS structures with various Al gate thicknesses were determined as a function of discharge parameters and plasma exposure time. The results show the vulnerability of the oxide and its interface with Si to the plasma process. The creation of a large amount of defects in the form of a fixed oxide charge and interface states is found; the build-up plasma damage process is very rapid, particularly during the first 5-30 s of the plasma exposure time and it depends essentially on the Al gate thickness (the thinner Al gate exhibits stronger plasma sensitivity). It is established that build-up damage depends on the plasma uniformity, but the non-uniformity is not the only nor the dominating factor. The nature of the process-induced defects and the influence of plasma components are also discussed.